U.S. Pat. Nos. 3,684,665 and 3,763,015 disclose the recovery of styrene from such hydrocarbon fractions. According to the processes disclosed in these references, fractions are recovered from the complex hydrocarbon mixtures by separating lighter hydrocarbons as the top fraction in one column and separating the heavier hydrocarbons as a bottom fraction in the second column. Xylenes and styrene may be separated from these fractions by means of extractive distillation.
Cracking of hydrocarbons for the preparation of ethylene and propylene is a well known process and is effected at a very large scale. Modern cracking plants have capacities for the production of several hundreds of thousands of tons of ethylene per year.
A more detailed general review of various cracking processes is given in report No. 29 `Ethylene` by Shigeyoshi Takaoka of August 1967 and its supplement A report No. 29A `Ethylene-Propylene` of March 1971 of the Stanford Research Institute; Menlo Part, Calif. These reports discuss several of the more common types of hydrocarbon cracking processes wherein direct heat is used for the cracking. These reports are hereby incorporated by reference.
The Stanford Research Institute reports are merely exemplary. The reaction conditions and the amounts of reaction products and fractions included in the discussion of the flow sheets in the reports will vary depending on the process parameters selected and the composition of the feed streams. The present process while limited to the recovery of styrene from the reaction products of hydrocarbons cracked by direct heating does fall within the overall scope of the disclosure in the Stanford Research reports.
The nature of the starting material which is cracked, the reaction time and the reaction temperature, influence the composition of the cracked products and in particular, the yield of ethylene and propylene. If naphtha is used as a starting material for the cracking process, ethylene yields of 20 to 35% by weight based on the starting material can be obtained at cracking temperatures ranging between 700.degree. to 900.degree. C. Similar yields can also be obtained from other starting materials, including lighter hydrocarbons such as ethane and propane, and from heavier fractions such as kerosene or gas oil, by choosing the appropriate process conditions.
Ethylene and propylene are not the only products of these cracking processes. Various other hydrocarbons, such as methane, ethane, propane, butenes and butanes, fractions consisting of hydrocarbons with at least 5 carbon atoms and having a maximum boiling point ranging between 180.degree. to 250.degree. C., the so-called cracked gasoline, and the heavier fraction, the quench oil, are produced. In addition, the reaction mixture from the cracking plant also contains slight amounts of hydrogen. Generally, both the quench oil and the cracked gasoline contain aromatic compounds. In most cracking processes, the amounts of ethane produced are so large, that after separation of other components, the ethane component is recycled to the furnace or to a separate ethane cracker to be employed as a material to be cracked.